To accommodate traffic increase in networks, optical networks that use optical fibers are being used in core networks. The optical network allows for very high capacity communication by employing a wavelength division multiplexing (WDM) technology that multiplexes a plurality of different wavelengths in a single optical fiber.
A WDM network includes a plurality of nodes, which are referred to as optical cross connects (OXCs), and optical fibers that provide connections between the nodes. The OXC is capable of creating a logical communication path, which is referred to as a lightpath, as an optical layer in between arbitrary nodes. The lightpath is actualized by setting a wavelength that may be commonly shared between two nodes. Setting a single lightpath allows creation of a logical high capacity communication path that corresponds to a capacity of one wavelength between two nodes.
An upper layer of the WDM network includes an internet protocol (IP) layer. In the IP layer, information from a user is transmitted and received on a packet by packet basis. In the WDM network, a router is connected to each OXC. A packet signal arrived at each OXC is analyzed by the router and forwarded to a forwarding destination OXC.
A network including the optical layer and the IP layer is referred to as an IP/WDM network. In the IP/WDM network, packets transmitted from users are aggregated by the router. Aggregated packets are forwarded to other nodes using the optical layer, which is a lower layer when viewed from the IP layer, as a transmission path.
Even if one or other nodes exist physically in the transmission path between two nodes, it is recognized that the two nodes are adjacent to each other when they are viewed from the IP layer in a case where the lightpath is set up therebetween in the optical layer. Each of the other nodes existing between the two nodes, between which the lightpath is set up, performs a process for switching an optical signal path by the OXC. The optical signal processed by the OXC does not go through the router. Accordingly, optical signals in the optical layer are not processed on a packet by packet basis. Such a process of the optical signals, which does not involve the router, is referred to as a cut-through.
Compared to forwarding by the lightpath, a complicated process is typically performed in the IP layer. The process includes operations such as converting an optical signal received at the OXC to an electrical signal, identifying a forwarding destination for each packet at the router based on the electrical signal converted, and forwarding the packet to the forwarding destination identified. The more complicated the process becomes, the higher the router performance is desirable. Accordingly, a network cost rises as the volume of signal processing in the IP layer increases.
An IP/WDM network design is carried out by selecting one or more lightpaths in such a way that a cost of the whole network is reduced by using, as inputs, a set of start point nodes, end point nodes, and traffic volume information flowing between the respective pairs of nodes, and a physical topology of the optical layer. A topology that is logically constructed with one or more lightpaths is referred to as a logical topology. Mukherjee (B. Mukherjee, “Optical WDM Networks”, Springer, 2006) teaches a maximizing multi-hop traffic (MMT) algorithm as an evaluation technology of the IP/WDM network design. In the MMT algorithm, the logical topology that reduces the cost of the whole network is obtained by setting the lightpath in between two nodes where the traffic volume is large as well as the number of relaying routers (hop count) is large.